The use of highly brilliant synchrotron radiation provided by third-generation storage rings allows precise structure analyses of crystalline materials by means of X-ray diffraction. However, a variety of experiments is limited on the part of presently available detectors. The possibility to measure simultaneous position-, energy-, and time-resolved diffraction signals of the sample is offered for the first time by pnCCD systems developed at the Max-Planck-Institute Halbleiterlabor. The principle of a pnCCD is based on the electron-hole pair creation within a fully sideward depleted silicon layer by incident X-rays.
The present work describes the general limitations of pnCCDs used for diffraction experiments with white synchrotron radiation. In that respect, both the achievable position, energy, and time resolution of single photons in the X-ray spectroscopy mode and the detector response in the integration mode are of considerable importance. The energy resolution of the pnCCD is investigated in the energy range between 6 keV and 20 keV by means of X-ray fluorescence spectroscopy. The experimental determination of the maximum number of storable charges per pixel, which defines the dynamic range of the detector, relies on analyses of the count rate behavior in the case of single-pixel illumination with intense monochromatic X-rays.
In the second part of the work, the potential of pnCCD systems for structure analyses is demonstrated within the scope of energy-dispersive Laue diffraction experiments on tetragonal hen egg-white lysozyme crystals. The evaluation of the simultaneously measured spot positions and energies enables a calculation of the conventional unit cell from a single Laue pattern. This result is independent of the sample orientation, applicable to polycrystalline materials without restriction, and can be obtained without additional information about the sample. In this sense, pnCCD systems allow a fast characterization of polycrystals by means of white-beam Laue diffraction. The determination of integrated Bragg peak intensities is based on a statistical analysis of pile-up events generated by spatially overlapping charge clouds associated with different photons. The developed methods use the fact that the distribution of detected photon numbers is given by Poisson statistics. In this way, structure-factor amplitudes can be extracted from the pnCCD data sets and used for a structure refinement on an atomic level.